For string waves at the ends of strings there is a reversal of phase and it plays an important role in producing resonancein strings. The phase of the reflected sound waves from hard surfaces and the reflection of string waves from their ends determines whether the interference of the reflected and incident waves will be constructive or destructive. Reflection of waves in strings and air columns are essential to the production of resonant standing waves in those systems. The doubling of pressure gives a 6 decibel increase in the signal picked up by the microphone. This is used in pressure zone microphones to increase sensitivity. It also means that the sound intensity near a hard surface is enhanced because the reflected wave adds to the incident wave, giving a pressure amplitude that is twice as great in a thin " pressure zone" near the surface. This can lead to resonances called standing waves in rooms. The reflected waves can interfere with incident waves, producing patterns of constructive and destructive interference. The same behavior is observed with light and other waves, and by the bounce of a billiard balloff the bank of a table. The reflection of sound follows the law "angle of incidence equals angle of reflection", sometimes called the law of reflection.
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